1. Field of Invention
This invention relates to a noise cancelling watertight sound transducer which is useful as a microphone or earphone. A hollow shell contains an inertial mass and an output signal is produced when the shell vibrates relative to the mass either as a result of external sound waves striking the shell, or as a result of a magnetic or piezoelectric actuator within the enclosure attempting to move the mass and thereby causing the shell to vibrate.
2. Description of the Prior Art
Acoustic transducers useful in underwater applications are known in the art. Few of such transducers, however, cancel noise such that random external pressure variations do not appear in the output as noise. Further, even fewer are small enough or sufficiently portable to be useful as an earphone or microphone for underwater applications such as by a scuba driver.
U.S. Pat. No. 4,797,863 to Gonzalez et al describes and underwater transducer with an annular, spring steel band supporting a fabric diaphragm. A flexible piezoelectric cable transducer is attached adjacent the band. Acoustical disturbances cause vibrations of the diaphragm deforming the cable to produce an electrical output signal. The present invention is less costly, simpler, lighter and less cumbersome than this prior art device.
U.S. Pat. No. 4,763,307 to Massa describes a sealed underwater acoustic transducer with a massive vibratory member in contact with the water actuable in response to a power supply to produce oscillatory vibrations in the water. This device is heavy and complex, and is not useful as a portable microphone or earpiece.
A more useful device is shown in U.S. Pat. No. 3,764,966 to Abbagnaro which comprises an underwater earphone assembly with a cylindrical casing sealed on both sides, one having a vibratory diaphragm and the other side a flexible rubber diaphragm. Electromagnetic energizing coils inside the casing cause the vibratory diaphragm to move. The device is filled with oil which damps the response of the device and also reduces its efficiency. The device's primary problem is that the vibratory diaphragm is clamped to the casing around its periphery and its greatest deflection is at its center, which creates distortions in the output.
The present invention is substantially simpler, lighter and less complex and expensive than the prior art, and is specifically adapted for use in underwater applications such as an earpiece or throat microphone for persons such as scuba peronnel. The benefits of the device are produced primarily by its use of a rigid hollow shell, the shell being able to withstand water pressure of 300 feet or more. An inertial mass is located inside the hollow shell and mounted such that it can move freely along a predetermined axis. When used as a microphone, the shell vibrates in response to the acoustic pressure due to the voice input, and because the inertial mass attempts to remain in its original position an electromagnet or piezoelectric element inside the shell will produce an audio frequency output signal. In this application the shell may be mounted by a suspension such as foam in a casing open to sound vibrations front and back. When used as an earphone, the electromagnet or piezoelectric element attempts to move the inertial mass, but because it resists movement the force is translated to by the shell which vibrates at the audio frequency.